A required drive force is changed while a vehicle is running, and an energy efficiency of a prime mover is deteriorated if the prime mover is operated at an operating point (or a driving point) away from a predetermined optimum operating point. Therefore, a transmission is arranged on an output side of the prime mover, and a drive force is adjusted in accordance with a speed ratio while controlling a rotational speed of the prime mover in an optimally energy efficient manner. In general, an automatic geared transmission adapted to change a speed ratio stepwise and a continuously variable transmission adapted to change a speed ratio continuously are used in vehicles. In the vehicle having this kind of transmission, a torque converter is disposed between the engine and the transmission to keep the engine driving in a stopping vehicle.
As known in the conventional art, the torque converter is adapted to transmit a torque by rotating a turbine by a spiral oil flow created by a pump impeller. In the torque converter, the torque is amplified by altering a direction of the oil flow returning to the pump impeller by a reaction force of a stator under the condition that a speed difference between the pump impeller and the turbine is large (i.e., if a speed ratio is small). Accordingly, a creep torque is generated under the condition that the speed ratio is small. Nonetheless, the torque applied to the transmission connected to an output side of the torque converter is changed responsive to the speed ratio of the torque converter. Therefore, as described in Japanese Patent Laid-Open No. 11-325232, an oil pressure applied to engagement elements of the transmission such as a clutch and a brake is controlled to control a torque transmitting capacity thereof in accordance with a condition of the torque converter.
The control system taught by Patent Laid-Open No. 11-325232 is applied to a vehicle in which an automatic geared transmission is connected to an output side of an internal combustion engine to control hydraulic pressure applied to a clutch of the transmission. In the vehicle of this kind, an engine torque starts to be changed after a lag behind an accelerating operation, and a delay in hydraulic response is also caused. Therefore, a hydraulic pressure applied to the clutch may be changed after a lag behind an increase in a turbine torque of the torque converter interposed between the engine and the transmission. In order to solve such disadvantage, according to the teachings of Japanese Patent Laid-Open No. 11-325232, an engine torque is calculated based on parameters representing an engine load such as an opening degree of an accelerator and an air-intake, and an engine speed, and an input torque of the clutch is calculated based on the calculated engine torque and the engine speed. That is, the engine torque is changed in accordance with the engine speed and a physical amount of the air-intake. Therefore, the engine torque is calculated first of all based on the parameters representing the engine load and the engine speed. A turbine torque of the torque converter can be calculated based on an input torque, a capacity coefficient, a speed ratio and so on. Therefore, a clutch torque corresponding to the turbine torque is then calculated based on the engine torque corresponding to the input torque and the engine speed corresponding to the input speed. A torque transmitting capacity of the clutch is increased with an increase in the hydraulic pressure. Therefore, if the input torque is known, the hydraulic pressure can be calculated based on the input torque.
In turn, Japanese Patent Laid-Open No. 2005-291174 describes a torque control device configured to estimate an engine torque. According to the teachings of Japanese Patent Laid-Open No. 2005-291174, the estimation torque is calculated using a torque converter characteristic defined based on a relation between an input shaft speed and an output shaft speed. Specifically, the estimation torque is calculated by multiplying a coefficient of capacity of a torque converter by a speed ratio and a square of the input speed, and by adding inertia torques of an engine and the torque converter and a loss torque of an auxiliary to the product. In addition, according to the teachings of Japanese Patent Laid-Open No. 2005-291174, an engine torque characteristic is learnt by comparing the estimation torque using the torque converter characteristic with the estimation torque based on an air quantity, and such learning of the engine torque characteristic is inhibited provided that a predetermined condition is satisfied. For example, the learning of the engine torque characteristic is inhibited if a change rate of an input speed or an output speed is large.
Thus, according to the teachings of Japanese Patent Laid-Open No. 11-325232, the parameters representing the engine load can be detected in short time so that the above-explained time lag can be ignored. Therefore, the hydraulic pressure applied to the clutch can be controlled without delay. However, the control taught by Japanese Patent Laid-Open No. 11-325232 is carried out utilizing a predetermined relation between the parameter such as a throttle opening or an air intake and an output torque. That is, the control delay of the clutch can be eliminated by the control system taught by Japanese Patent Laid-Open No. 11-325232, however, the pressure would be applied to the clutch excessively or insufficiently if the engine torque or the input torque of the clutch cannot be detected accurately. In the conventional vehicle, so-called “stop and start” control may be executed optionally to improve fuel economy upon satisfaction of a predetermined condition. According to the stop and start control, specifically, the engine is stopped when the accelerator is closed completely, and the engine is restarted when such condition is eliminated. To this end, the torque converter is disconnected from the transmission by disengaging the clutch during stopping the engine, and the clutch is engaged when the engine is restarted. During restarting the engine, a combustion state in the engine is unstable until a complete explosion of fuel is achieved, therefore, an air fuel-ratio is controlled to be richer than that under the situation in which the engine rotates autonomously. That is, the relation between the throttle opening or an air intake and the output torque is deviated from the predetermined relation. Therefore, the response delay can be eliminated by the control system taught by Japanese Patent Laid-Open No. 11-325232 but the hydraulic pressure applied to the clutch cannot be controlled appropriately with respect to the input torque. For this reason, shocks of the clutch may be caused and an excessive slippage may be caused thereby damaging the clutch.
As described, the torque control device taught by Japanese Patent Laid-Open No. 2005-291174 is configured to learn and inhibit the engine torque characteristic depending on a situation. To this end, the engine torque characteristic is estimated based on the characteristic of the torque converter, however, the estimated value will not be reflected as an actual torque on the control. As described, the torque thus estimated using the torque converter characteristic is compared with the estimated torque based on an air quantity. However, the relation between the estimated torque and the engine load differs under the condition that the engine is operated unstably during restarting. In this case, therefore, the engine torque cannot be estimated accurately and the control taught by Japanese Patent Laid-Open No. 2005-291174 cannot be carried out.